Referring to FIG. 1, a conventional energy storage system includes a flywheel energy storage device 91 and a driver device 92. The flywheel energy storage device 91 includes a flywheel 911, and a motor 912 that can also be used in reverse as a generator. Referring to FIGS. 1 and 2, the motor 912 includes a stator (not shown), a rotor (not shown) that is coupled to the flywheel 911, and three coils (R, S, T) that are wound around the stator.
As shown in FIG. 2, the coils (R, S, T) cooperatively form a star configuration 913 that has a central terminal (Np) and three end terminals (U, V, W), with each coil (R, S, T) coupled between a respective end terminal (U, V, W) and the central terminal (Np). Referring to FIGS. 1 and 3, the driver device 92 is electrically powered by a direct current (DC) power source 93, and is for driving the motor 912. As shown in FIG. 3, the driver device 92 includes three legs 921-923. Each leg 921-923 includes a first switch (U+, V+, W+) and a second switch (U−, V−, W−) which are coupled to each other, and a common node of which is coupled to a respective end terminal (U, V, W). The first switches (U+, V+, W+) are coupled to each other, and a common node thereof is used to be coupled to a positive terminal of the DC power source 93. The second switches (U−, V−, W−) are coupled to each other, and a common node thereof is used to be coupled to a negative terminal of the DC power source 93. Referring to FIGS. 1 to 3, each of the first and second switches (U+, V+, W+, U−, V−, W−) alternates between conduction and non-conduction in such a way that the coils (R, S, T) are repeatedly excited by a DC supply voltage (Vdc) which is supplied by the DC power source 93. As a consequence, the coils (R, S, T) cause the rotor to rotate (i.e., performing electrical to mechanical conversion), and the rotor causes the flywheel 911 to rotate and store mechanical energy. In case the driver device 92 is no longer coupled to the DC power source 93, the flywheel 911 causes the rotor to rotate and releases the energy stored therein, and the rotor causes the coils (R, S, T) to provide electrical energy (i.e., performing mechanical to electrical conversion).
Upon each of the first and second switches (U+, V+, W+, U−, V−, W−) becoming non-conducting, a counter electromotive force (CEMF) is developed across one of the coils (R, S, T) that is coupled to the switch (U+, V+, W+, U−, V−, W−). For example, when the first switch (U+) and the second switch (V−) both conduct, the DC supply voltage (Vdc) is applied across the coils (R, S) to excite the coils (R, S) as shown in FIG. 2, and the coils (R, S) store electrical energy. Upon the first switch (U+) and the second switch (V−) both becoming non-conducting, the CEMFs (e1, e2) are respectively developed across the coils (R, S) as shown in FIG. 4, and the coils (R, S) release the energy stored therein. The CEMFs (e1, e2) continue until the energy stored in the coils (R, S) is fully released. The respective CEMFs across the coils (R, S, T) directly impact the DC power source 93, resulting in variation of the DC supply voltage (Vdc). The energy released by the coils (R, S, T) is dissipated as heat by the motor 912 and the legs 921-923, resulting in energy loss and temperature increase of the motor 912 and the legs 921-923. In order to alleviate the aforesaid phenomena, a capacitor (Cd) is included in the conventional energy storage system and is coupled between the common node of the first switches (U+, V+, W+) and the common node of the second switches (U−, V−, W−) as shown in FIG. 3 for stabilizing the DC supply voltage (Vdc), and for storing the energy that is released by the coils (R, S, T) and that flows through respective parasitic diodes (D) of the first and second switches (U+, V+, W+, U−, V−, W−) to the capacitor (Cd). The energy stored in the capacitor (Cd) assists in electrically powering the legs 921-923. However, the capacitor (Cd) can only store a small portion of the energy released by the coils (R, S, T), and a remaining portion of the energy released by the coils (R, S, T) is still dissipated as heat by the motor 912 and the legs 921-923. Therefore, the conventional energy storage system has poor energy saving performance, and a cooling system is required to cool the motor 912 and the legs 921-923.
Moreover, an internal combustion engine uses fuel to cause reciprocal movement of a piston of a cylinder thereof.